1. Field of the Invention
The present invention generally relates to the operation of instrumentation within a wellbore. More particularly, the invention relates to a separable plug for use with a wellbore tool.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. After drilling a predetermined depth, the drill string and the drill bit are removed, and the wellbore is lined with a string of steel pipe called casing. The casing provides support to the wellbore and facilitates the isolation of certain areas of the wellbore adjacent hydrocarbon bearing formations. An annular area is thus defined between the outside of the casing and the earth formation. This annular area is typically filled with cement to permanently set the casing in the wellbore and to facilitate the isolation of production zones and fluids at different depths within the wellbore. Numerous operations occur in the well after the casing is secured in the wellbore. All operations require the insertion of some type of instrumentation or hardware within the wellbore. For instance, wireline logging tools are employed in the wellbore to determine various formation parameters including hydrocarbon saturation.
Early oil and gas wells were typically drilled in a vertical or near vertical direction with respect to the surface of the earth. As drilling technology improved and as economic and environmental demands required, an increasing number of wells were drilled at angles which deviated significantly from vertical. In the last several years, drilling horizontally within producing zones became popular as a means of increasing production by increasing the effective wellbore wall surface exposed to the producing formation. It was not uncommon to drill sections of wellbores horizontally (i.e. parallel to the surface of the earth) or even “up-hill” where sections of the wellbore were actually drilled toward the surface of the earth.
The advent of severely deviated wellbores introduced several problems in the performance of some wellbore operations. Conventional wireline logging was especially impacted. Wireline logging utilizes the force or gravity to convey logging instrumentation into a wellbore. Gravity is not a suitable conveyance force in highly deviated, horizontal or up-hill sections of wellbores. Numerous methods have been used, with only limited success, to convey conventional wireline instrumentation or “tools” in highly deviated conditions. These methods include conveyance using a drill string, a coiled tubing, and a hydraulic tractor. All methods require extensive well site equipment, and often present operational, economic, and logistic problems.
Another problem that affects both a deviated wellbore and a vertical wellbore occurs when the wellbore contains a high percentage of water relative to the hydrocarbons in the surrounding formations. In this situation, fluid tends to collect and remain static proximate the lowest point of the wellbore because there is not enough hydrocarbon formation pressure to move the fluid. For instance, fluid tends to collect at the junction between the vertical portion and the deviated portion in a deviated wellbore. Without fluid flow, production logging tools can not operate properly to collect data. To overcome this problem, some form of artificial lift is typically employed to move fluids through the wellbore, such as a submersible pump. The increased velocity of the fluid provides an adequate flow rate for the logging tool to operate.
Generally, the submersible pump is run into the wellbore on production tubing with a Y block between the production tubing and the submersible pump. The Y block allows the pump to be turned on and the well produced while leaving an access point to the wellbore for logging tools. Typically, the access point is a smaller string of tubing attached to the Y block which is run along side the submersible pump. In operation, a logging tool is conveyed through the production tubing attached to a string of coiled tubing. As the logging tool passes through the Y block and the smaller string of tubing, a plug attached to the string of coiled tubing lands in a seat formed in the smaller string of tubing. The plug seals off the smaller string of tubing while allowing the string of coiled tubing and the logging tool to continue to travel into the wellbore. Although coiled tubing may be used in deviated wellbores, the coiled tubing and associated injector equipment are still physically large and present drawbacks similar to those encountered with drill string conveyed systems.
A need therefore exists for a reliable and operationally efficient system to convey and operate wellbore tools, like logging tools, in wellbores which are deviated from the vertical.